Since Luke learned about the Force and Obi-Wan trained him, he should naturally have been very interested in everything related to the Jedi Order. Did he never look at pictures of the old Jedi and their Grand Master, Yoda? Were there no records of any kind that he studied with incredible enthusiasm?

Only 20 years had passed since the Old Republic died, and a lot of the rebels around Luke were older than 35, so they must have had vivid memories of the Old Republic and the famous Jedi. One of them should have told Luke what species Yoda was from (even if it has no official name) and what he looked like.

But Luke went to Dagobah without the slightest clue. How could that happen?

@Olaf - Yes, but Kenobi was dead -- who (or what) was left to teach him? The purge had eliminated the majority of Jedi culture, as it apparently had been intended to; see the whole scene in ANH with Vader being mocked for his adherence to a forgotten 'religion.'
– K-H-WNov 24 '13 at 20:05

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@KHW - And still it is very unlikely that all information has been lost. For example, Leia grew up as the adopted daughter of senator Organa who knew Yoda personally. She must have heard and read the story that shaped the galaxy many times, and she had three years to tell Luke. And yet he didn't know anything at all about the old master of the Jedi. This is not about teaching, just plain basic information.
– OlafNov 24 '13 at 21:39

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@Olaf - Maybe.. But, I get the impression it was a systematic purge, attempting to wipe the Jedi from both reality and history. Luke had a lot more to do during that time; the Rebel Alliance was still not in control, and, as you see on Hoth, he was acting, to some degree, as one of their soldiers. I think, had he had the time, he could have found answers; he just didn't have enough spare time to dig for information that had been suppressed. Also, Leia was kept intentionally ignorant of many things while growing up to avoid drawing attention. (See the EU books about her childhood.) Just MHO.
– K-H-WNov 24 '13 at 21:56

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Star Wars occupies a very strange technological landscape. They have FTL travel/communication and extremely advanced robots with strong AI, but they don't have the internet or P2P. Otherwise, he could have just downloaded a torrented copy of Wookieepedia on his Galaxy Nexus and learned what Yoda looked like.
– Lèse majestéNov 25 '13 at 0:04

Luke did NOT discuss anything about Dagobah or Yoda with any "older Rebels" because he:

had no time - he was first freezing his butt on Hoth, then recovering from said freezing in a hospital, then Empire Striked Back and he was fighting AT-ATs; and then he was evacuating in his single-pilot X-Wing.

Didn't even know if he was hallucinating in the first place. He didn't decide to go to Dagobah till he was in the X-Wing

He had no reason to suspect that Yoda was a famous Jedi known to random rebels. Jedi were rare and mysterious and pretty much nobody discussed them in the Empire.

Also, as discussed in numerous places in EU, there was no "Jedi culture" for Luke to study before Hoth - leaving aside the fact that he was fighting a Rebellion, nearly ALL information on Jedi order was destroyed when Order 66 was implemented and in subsequent years. A lot of EU material centers on Luke's VERY VERY difficult search for any information at all about Jedi - and that was when he was both an accomplished Jedi Master AND after Rebels won.

Still, within the 3 years he would have naturally inquired about the whole story of the empire and the rebellion. And the name "Yoda" would be one of the first to be mentioned. By Chewbacca and others.
– OlafNov 27 '13 at 9:11

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Why would Luke ask Chewbacca about the fall of Kashyyyk? You are correct that it was odd that any of the several Rebels who knew Yoda personally or at least knew of him never spoke to Luke about him, but they had good reasons for doing so. Skywalker wasn't the only wannabe Jedi in the Rebel Alliance, though he was the most prominent. He also was being personally hunted by Lord Vader, with all the potential for capture and torture that entailed. Chewbacca did not know if Yoda had survived after he left Kashyyyk, and keeping his mouth shut about Yoda might just keep Yoda alive. Ditto for others.
– James SheridanDec 8 '13 at 13:00

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@JamesSheridan: Also, it's a little hard for Luke to have casual conversations with Chewbacca. "Hey Chewie, did you ever know any Jedi?" "Wrroaaagh, ruuughh rooarrrh." "OK, I need someone to translate that. Where did Threepio go?"
– Royal Canadian BanditMar 24 '15 at 10:10

The simplest explanation would be that Luke did try to find out about the Jedi, but didn't know what he was looking for.

From Luke's perspective, Yoda is one of many members of the Jedi Council. All of them are presumed dead and several belong to alien races. He didn't have any reason to research Yoda specifically.

Obi-Wan could have told Luke on the Millennium Falcon, but was waiting until the time was right. If Luke knew that Yoda was alive, he might have let something slip, which could have gotten back to the Empire, who would have gone looking for Yoda. Obi-Wan and Yoda wanted time to train Luke on Dagobah without being disturbed, so Obi-Wan didn't reveal Yoda's location until he wanted Luke to go there right away.

Exactly why Obi-Wan waited a few years after his own death to tell Luke is not explained. Maybe it took a while for him to master Force-ghosting well enough to communicate with Luke. Speculating wildly, the fact that Luke himself was near death at the time might have made it easier for Obi-Wan to make the initial contact.

A few other significant factors:

While there might have been records of Yoda's appearance on Coruscant, they would not necessarily be reproduced in the remote backwater systems where the Rebellion was hiding.

The Empire had actively been trying to suppress knowledge of the Jedi for twenty years. It would be like trying to research the victims of Stalin's purges in the USSR, while Stalin was still alive. You can't just walk into the public library and look them up, and asking about them (in systems where the Empire holds any influence at all) would likely draw unwelcome attention. If Luke did find anything, it would be difficult to separate reality from Imperial propaganda.

Luke was part of a Rebellion which was fighting desperately for its existence. He might not have had time to do much research into the Jedi. By way of comparison, Han Solo didn't have time to visit Jabba the Hutt and pay him off to remove the price on his head -- and avoiding bounty hunters might be considered more urgent than researching (presumed) long-dead Jedi Masters.

It's worth noting too that Bail Organa was probably the member of the Rebellion who knew Yoda best (having worked out the adoption of Leia with him), and he was busy being dead after the Empire destroyed Alderaan. There's no evidence that anybody else in the Rebellion knew Yoda except by reputation.
– ThunderforgeDec 1 '15 at 2:04